Spanning-Tree Priority

Question

We have a network of 5 Dell 5224s, 1 6024 and a few linksys wireless routers. When STP completes it has one of the linksys set as root bridge for all the Dells. We've been having some connectivity issues and think this might have something to do with it.

Would changing the priority lower on the uplink ports (24 is a fiber uplink) force STP to choose the 6024 as root?

DELL-Cuong N. · Answer

Yes, if you want to control who will become the designated root bridge you will need to make sure that you set the bridge priority for the switch (this setting is global to the 6024 switch not at a port level - 'spanning-tree priority ') to be lower then any other bridge priority for other switches in this network.  The bridge priority combine with the MAC address of the switch is used to create the bridge identifier.  The switch with the lowest bridge identifier is always elected the root bridge.  This tends to be somewhat arbitrary because of the way MAC addresses are assigned to vendors so by using the bridge priority you can control the bridge identifier and therefore how spanning tree will layout in your network.  Since the bridge priority is the most significant bits of the bridge identifier, if you set this priority lower on the 6024 then any other switch in the network it will cause the 6024 to be elected as the root bridge. Couple of things you should take into consideration: Make sure the switch you want to be root is the fastest switch you have (in this case 6024 is a good choice) since the root tends to take the brunt of most of the traffic so lower performing switch will cause poor performance for the entire network. Make sure that the root bridge is topologically central to your network since traffic will tend to converge at the root. Make sure the root bridge is connected to the fastest links in your LAN. Cuong.

Networking General

Spanning-Tree Priority

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